In recent years, social demand with respect to energy conservation and resource saving has increased. In order to reduce fuel consumption of automobiles, demand has arisen for various rubber materials having low-loss performance and breakage resistance for providing tires of high durability.
One generally employed technique for improving rolling resistance of a tire is use of a low-heat-buildup rubber composition. In order to attain this performance, use of a modified polymer into which a functional group having interaction with a filler has been incorporated is very effective means.
When a technique for improving the rolling resistance of tire based on reduction of the amount of reinforcing filler is employed, durability of the tire is unavoidably impaired. However, it has already been known that both low-loss performance and durability can be attained by incorporating a modification group into the polymer to be employed. The interaction between the reinforcing filler and the polymer is known to vary depending on the type of the modification functional group to be incorporated into the polymer, and the performance of the tire produced thereform is also known to vary.
Hitherto, in order to produce such a low-heat-buildup rubber composition, a variety of modified rubbers containing silica or carbon black serving as a filler have been developed. Among the techniques for producing such modified rubbers, there have been proposed particularly effective approaches in which the polymerization active site of a conjugated diene (co)polymer produced through anionic polymerization in the presence of an organic lithium is modified with an alkoxysilane derivative having a functional group interacting with a filler (see, for example, Patent Document 1 or 2).
When a reinforcing filler is incorporated into a rubber composition employing the modified polymer produced through the aforementioned method, heat-buildup-suppressing performance is obtained. However, in this case, a volatile organic compound (VOC); i.e., a volatile alcohol, is released from alkoxysilane contained in the modified conjugated diene (co)polymer during kneading, hot-rolling, or extrusion of an unvulcanized rubber composition, thereby generating air bubbles in an extrusion product of the unvulcanized rubber composition, resulting in a drop in processability (workability of unvulcanized rubber composition), which is problematic. In addition, the volatile organic compound (VOC) is not preferred in the working environment.
Studies conducted by the present inventors previously revealed that a modified polymer having a primary amino group exhibits high interaction with carbon black and provides a rubber composition having excellent low-loss performance. Regarding incorporation of the primary amino group into a polymer, for example, Patent Document 3 discloses a rubber ingredient containing a conjugated diene polymer having a primary amino group and an alkoxysilyl group, and a rubber composition containing carbon black having an N2SA of 30 to 100 m2/g.
However, when the above technique is employed, alkoxysilyl groups remaining in the polymer might be released as alcoholic matter to the atmosphere during polymer blending, which is also problematic.
Meanwhile, in recent tires, particularly run-flat tires, there is provided a side-reinforcing layer formed of a rubber composition or a composite of a rubber composition and fiber or the like in order to enhance the toughness of the conventional sidewall part (see, for example, Patent Document 4).
When a tire is in a running state under reduced internal pressure thereof due to blowout or the like (i.e., run-flat running state), the sidewall and the bead filler of the tire considerably deform and generate heat. In the progress of heat generation, the temperature is elevated to 200° C. or higher in some cases.
Under such circumstances, even a tire having a side-reinforcing layer is broken due to a load exceeding the breakage limit of the side-reinforcing layer and the bead filler.
One technique for prolonging the time before such breakage occurs includes incorporating a large amount of sulfur into a rubber composition for forming a side-reinforcing layer or a bead filler, to thereby elevate elasticity of the rubber composition and suppress deformation of the sidewall part and the bead filler of the tire. However, when the technique is employed, the rolling resistance of the tire under normal running conditions increases, problematically elevating fuel consumption.
Patent Document 5 discloses that a side-reinforcing layer and a bead filler of a tire are formed from a rubber composition comprising a variety of modified conjugated diene-aromatic vinyl copolymers and a heat-resistance-improver.
Patent Document 6 discloses that a side-reinforcing layer and a bead filler of a tire are formed from a rubber composition comprising a specific conjugated diene-based copolymer and a phenolic resin.
All these techniques are developed for the purpose of increasing elastic modulus of the rubber composition for forming the side-reinforcing layer and the bead filler and preventing the elastic modulus from decreasing at high temperature. Therefore, the durability of run-flat tires is remarkably improved, but rolling resistance under normal running conditions is considerably impaired.
Thus, there is demand for a rubber composition which exhibits excellent heat-buildup-suppression performance, which can enhance both durability in running after blowing out as well as low rolling resistance under ordinary running conditions, and which is suitable for forming a side-reinforcing layer and a bead filler.
Patent Document 1: Japanese Patent Publication (kokoku) No. Hei 6-57767
Patent Document 2: WO 03/029299, pamphlet
Patent Document 3: Japanese Patent Application Laid-Open (kokai) No. 2006-307095
Patent Document 4: Japanese Patent Application Laid-Open (kokai) No. Hei 11-310019
Patent Document 5: WO 02/02356, pamphlet
Patent Document 6: Japanese Patent Application Laid-Open (kokai) No. 2004-74960